One of the biggest problems is that oil plumes in the ocean are unpredictable. Variations in temperature and salinity can create stacked layers of water with differing densities, and that can means that the oil doesn't rise up from the bottom of the sea in as straight a line as you may imagine.

This pink experimental plume allows scientists to understand how injecting additives or surfactants into the oil can change the way ti behaves underwater—and might just help us avoid another Deepwater Horizon. [AGU via Fuck Year Fluid Dynamics]